OPV devices have the most different devices architectures. Typically they comprise at least one organic semiconducting layer between two electrodes. That organic layer can be a blend of a donor and an acceptor such as P3HT (poly3-hexyl-tiophene) and PCBM (phenyl Cn Butyric Acid Methyl Ester). Such simple device structures only achieve reasonably efficiencies if interfacial injection layers are used to facilitate charge carrier injection/extraction (Liao et al., Appl. Phys. Lett., 2008. 92: p. 173303). Other organic solar cells have multi-layer structures, sometimes even hybrid polymer and small molecule structures. Also tandem or multi-unit stacks are known (Ameri et al., Energy & Env. Science, 2009. 2: p. 347). From those, the multi-layer devices can be easier optimized since different layers can comprise different materials which are suitable for different functions. Typical functional layers are transport layers, optically active layers, injection layers, etc.
Optically active materials are materials with a high absorption coefficient, for at least a certain wavelength range of the solar spectra, which materials convert absorbed photons into excitons which excitons contribute to the photocurrent. The optically active materials are typically used in a donor-acceptor heterojunction, where at least one of the donor or acceptor is the light absorbing material. The interface of the donor-acceptor heterojunction is responsible for separating the generated excitons into charge carriers. The heterojunction can be a bulk-heterojunction (a blend), or a flat (also called planar) heterojunction, additional layers can also be provided (Hong et al, J. Appl. Phys., 2009. 106: p. 064511).
The loss by recombination must be minimized for high efficiency OPV devices. Therefore, the materials in the heterojunction must have high charge carrier mobilities and high exciton diffusion lengths. The excitons have to be separated at the heterointerface and the charge carriers have to leave the optically active region before any recombination takes place. For those reasons, only few organic materials are suitable to be used in the heterojunction. For instance, currently, there are no known materials which can compete with the fullerenes and their derivatives (e.g. C60, C70, PCBM, and so on) as acceptor in OPV devices.
Transport materials are required to be transparent, at least in the wavelengths wherein the device is active, and have good semiconducting properties. Those semiconducting properties are intrinsic, such as energy levels or mobility, or extrinsic such as charge carrier density. The charge carrier density can be extrinsically influenced, for instance, by doping the material with an electrical dopant.
Although in steady development, the choice of materials for OPV is still very limited, especially for optically active materials and for electron transport materials. Some highly efficient device structures employ TiO as electron transport and optical spacer with the disadvantage of being difficult to deposit (Simon et al., Int. J. of Mat. & Prod. Tech., 2009. 34: p. 469). Other devices use Fullerene C60 as ETL which is not transparent enough for functioning as an optical spacer. Other materials such as NTCDA, although transparent and with good semiconducting properties, are not morphologically stable and crystallize even at room temperature.
The new materials can also be employed in OLEDs, and in OTFTs.
Almost no organic electron transport material is available with suitable semiconducting, chemical, and thermal properties.